1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a method and apparatus for identifying and acquiring a wireless communications system that is more desirable than a current wireless communications system.
2. Description of the Related Art
A mobile device may have access to more than one wireless communications system in its current geographic region. The quality of the wireless services available to the mobile device will vary from system to system depending on the equipment used by each system, the features of the mobile device, the distance between the mobile device and local base stations, physical obstructions such as buildings and hills, and the volume of communications traffic on each system. The wireless communications systems may also support different multiple-access wireless communications protocols such as code division multiple access (CDMA), wideband code division multiple access (WCDMA), time division multiple access (TDMA), Advanced Mobile Phone System (AMPS) or Global System for Mobile Communications (GSM). In addition, the fees charged to a user of the mobile device often vary depending on the time of day, the duration of the connection with the wireless communications system and whether the mobile device is listed as a subscriber of the wireless communications system.
To assist with the selection of a wireless communications system, conventional mobile devices store data describing each known wireless communications system in a systems table. Each entry in the systems table includes a system identifier (SID), a network identifier (NID), and acquisition parameters including band, frequency and mode. The wireless communications systems are often grouped by geographic region, and sorted from the most desirable to the least desirable system in each region. The most desirable system in a particular geographic region is typically a subscriber system, but may also be a roaming system that offers the mobile device the best combination of low cost and high quality of service. Roaming systems offer wireless services to non-subscriber mobile devices, usually at a much higher fee than subscription services, and may be desirable when the mobile device enters a geographic region that is outside the coverage area of the mobile device's subscription service, when the subscription services are blocked or otherwise unavailable, or when the subscription services are only available at an unacceptably low level of quality.
When activated, a conventional mobile device attempts to acquire the most desirable wireless communications system that is available in the current geographic region. The mobile device may determine its current geographic region by acquiring a first wireless communications system and locating its corresponding entry in the systems table. In one approach, the mobile device maintains a table of the wireless communications systems that were recently used by the mobile device, and the mobile device attempts to acquire one of these systems upon activation. In another approach, the mobile device initially attempts to acquire the wireless communications systems that are known to be the easiest to locate. For example, some wireless communications systems in different geographic regions use the same acquisition parameters, increasing the likelihood that the mobile device will be able to acquire a system when using these parameters, regardless of the current geographic region. Once a wireless communications system is acquired, the mobile device searches for the SID/NID pair of the acquired system in the systems table to determine its corresponding geographic region. The mobile device then searches the systems table for the most desirable system in the identified geographic region and attempts to acquire that system. If the mobile device is unable to acquire the most desirable system, the mobile device steps through the other entries in the current geographic region, from the most desirable to the least desirable, until a wireless communications system is acquired.
The use of a systems table to acquire the most desirable system does not guarantee that the mobile device will be always be connected to the most desirable system that is available in its current geographic region. During system acquisition, for example, the most desirable system may have been temporarily blocked or weakened by physical obstructions, forcing the mobile device to acquire a less desirable system. Even if the most desirable system is initially acquired, the systems that are available to the mobile device may change as the position of the mobile device and its surrounding environment changes. To provide the user of the mobile device with the best combination of high quality and low cost, some mobile devices periodically identify and attempt to acquire a wireless communications system that is more desirable than the current wireless communications system used by the mobile device. In operation, the mobile device searches the systems table for wireless communications systems in the current geographic region that are more desirable than the current communications system. If more desirable systems are found, the mobile device switches away from the current communications channel and attempts to acquire one of the more desirable systems.
Although it is beneficial to communicate using the most desirable wireless communications system that is available to the mobile device in the current geographic region, the approach describe above has many drawbacks. For example, it is common for the mobile device to leave the current communications channel for up to 10 seconds every 60 seconds while attempting to acquire a more desirable system. The duration of time that the mobile device is away from the current communications channel will depend on many factors, including the number of searches that must be performed in order to acquire a more desirable system and the communications protocol used by each of the wireless communications systems. While the mobile device is searching for wireless communications systems on other channels, the mobile device may miss important pages, telephone calls and other incoming communications from the current wireless communications system. In addition, this frequent and prolonged switching away from the current wireless communications system increases the likelihood that the current system will be lost when the signal is weak. Further, the numerous acquisition attempts that are performed in this approach drain the battery power of the mobile device, reducing its standby time.
In view of the above, there is a need in the art for an improved method and apparatus for identifying and acquiring a wireless communications system that is more desirable than a current wireless communications system.